This invention relates to an apparatus for making highly dispersed oxides, mixed oxides and oxide mixtures of metals and/or metalloids hydrophobic by treating the oxide particles with volatilizable organosilicon compounds in the gas phase.
It is known to make highly dispersed oxides (active fillers), which have been obtained by reaction of metal or metalloid compounds or volatile compounds thereof in vapor form, hydrophobic at elevated temperatures in the presence of a hydrolyzing agent and, possibly, an oxidizing agent also. In this process, the oxides are rendered hydrophobic by reaction with a halogen-containing inorganic or organic compound.
Oxide aerogels are usually made by subjecting volatile compounds of metals or metalloids, particularly halides or gas mixtures containing the same, to the hydrolyzing influence of water vapor while the compounds are in the gaseous phase, whereby the resulting oxides, which are present in the aerosol state, form aerogels. These products are then isolated from the easily condensed, gaseous reaction products at a temperature above their dew points. The water vapor-forming gas mixtures can consist of combustible gas mixtures, particularly hydrogen-containing gas mixtures, or compositions which form such mixtures, and also of non-combustible gases, preferably oxygen-containing gases. The oxides obtained have a primary particle size of less than 150 m.mu., volatile halides, and preferably chlorides and fluorides, may be used as starting materials for this process.
In the preparation of mixed oxides, different metals or metalloids, or compounds thereof which are volatile, are introduced to the thermal reaction as gaseous mixtures simultaneously so as to cause the oxides to separate out in the form of mixed oxides. On the other hand, the preparation of so-called oxide mixtures is effected by subjecting different volatile reaction compounds to the pyrolytic treatment separately but jointly converting the different materials from the aerosol to the aerogel state, i.e. co-coagulating them, so that the oxides obtained are in the form of oxide mixtures. It is also possible to subject different and separate oxides to a mechanical treatment after their preparation for the purpose of combining them to form oxide mixtures.
If halogen-containing starting materials such as, for example, silicon tetrachloride or silicon tetrafluoride, are used in the thermal reaction, there are obtained products which, as a result of their high adsorption capacities, contain large amounts of hydrogen halide and, in addition, contain halogen directly bound to the metal or metalloid atom. These oxides are strongly acid in their reactions. Their hydrohalic acid content may, for example, amount to 0.1%, so that they have a pH value of about 1.8. These oxides exhibit predominantly hydrophilic properties. In another way there are produced finely dispersed silica particles in a wet process by precipitation.
For many purposes such as, for instance, working highly dispersed filler materials into organic media, it is desirable for the filler material to possess organophilic, that is, hydrophobic, properties. Organo-chlorosilanes and various other agents have been used for this in various processes. Thus, it is known to make pulverulent silicic acid hydrophobic by treatment with alkylchlorosilanes so as to form a coating thereon. In this case, however, the chlorosilane present on the surface of the silicic acid adsorbs water giving rise to the formation of hydrochloric acid. The silicic acid which has been made hydrophobic in this manner must be freed from the hydrochloric acid which has formed.
It has also been proposed to make powdery silicic acid hydrophobic with silicone oils. This involves suspending the dry, pulverulent silicic acid in an organic liquid.
Pyrogenic metal oxides which have free OH groups on their surfaces have also been treated with gaseous or readily vaporizable materials such as alcohols, formaldehyde and ketenes, the oxides undergoing etherification, esterification or acetate formation. This treatment has been carried out simultaneously with or subsequent to a hydrolysis with water or steam. In the esterification, there are obtained products which, similar to the relatively unstable products obtained in saponification, do not, in general, meet the requirements for stability in hydrophobic products. The esterification-modified products have not, therefore, achieved industrial importance as truly stable hydrophobic products.
It is further known to make highly dispersed oxides hydrophobic by treating them with silanes in vapor form with the agent for imparting hydrophobic properties being added directly after the formation of the oxide from the halide in the presence of steam and oxygen at a temperature below 500.degree. C. The process of making the oxides hydrophobic takes place in the presence of free hydrogen halide formed during the production of the oxides, the hydrogen halide being present in large amounts. The resulting products have a pH value which does not exceed a value of 2.0.
In the aforementioned procedures, no chemical reactions with the OH groups on the surfaces of the oxides take place but, rather, the reactions are only with the surface-adsorbed water so that fine, particulate oxides in a stable form are not obtained. A stable hydrophobic material can be obtained only when a chemical reaction is involved. Only highly dispersed oxides which have been made hydrophobic by means of a true chemical reaction do not undergo extraction, e.g., from carbon tetrachloride by shaking with water. Those products which are not formed by chemical reaction with the OH groups are extracted into the aqueous phase since the carbon tetrachloride dissolves the organic molecules which are merely adsorbed off the surfaces of the oxides.
Attempts to alter the properties of a precipitated metal or metalloid oxide by making the same hydrophobic through reaction of the OH groups present on the surfaces thereof have not been lacking.
Thus, in the German Pat. No. 1,163,784, there is described a process for the surface treatment of highly dispersed metal and/or metalloid oxides which have free OH groups on their surfaces. The oxides may be homogeneous oxides, mechanical mixtures, mixed oxides or oxide mixtures. The oxides are obtained by thermal decomposition of volatile compounds of the metals and/or metalloids in vapor form in the presence of hydrolyzing and/or oxidizing gases or vapors. They are treated in uncondensed form and as freshly obtained from their formation. Prior to the treatment for making the oxides hydrophobic, the oxides are freed as far as possible of halogens, hydrogen halide and adsorptively bound water in the absence of oxygen. The oxides are then homogeneously mixed with substances capable of imparting hydrophobic properties to them and capable of reacting with the OH groups. For this purpose, they are introduced into a continuously operated reactor, which is in the form of a vertical tubular oven and which operates with concurrent flow, together with small amounts of steam and, advantageously, with an inert carrier gas. The reaction chamber is heated to a temperature between 200.degree. and 800.degree. C. and, preferably, between 400.degree. and 600.degree. C. The resulting solid and gaseous reaction products are separated and the solid products are preferably deacidified and dried. Contact with oxygen is not effected until cooling to below about 200.degree. C. has occurred.
The surface treatment with the compounds which are capable of reacting with the OH groups must take place in the presence of small amounts of steam with the result that the thermally destroyed groups are re-formed. It is recommended that about 0.5 to 2.0 m mol of water be introduced for each 100 m.sup.2 of surface area of the oxide. The treating agent for the reaction is introduced in an amount dependent upon the surface area and the ultimate application. Highly dispersed silicic acid having a surface area of 200 m.sup.2 /g has about 1 m mol/g of free OH groups. This would indicate that, theoretically, 1 m mol/g of reagent should be introduced for reaction therewith. In practice, however, it is advantageous to use 1.5 m mol/g of reagent.
In accordance with the above prior art process, any compounds which will react with the OH groups such as, for instance, by etherification, esterification or acetal formation, may be used as reactants for the surface treatment.
Suitable reactants include alcohols, aldehydes, ketenes, alkylene oxides and the like. Particularly good results are obtained if the oxide is reacted with the halide corresponding to the treatment compound. The finished oxides possess organophilic properties and can be dispersed in organic media such as, for instance, lacquers, with advantageous results.
The known agents capable of imparting hydrophobic properties can be used in order to obtain such properties. Preferably, alkyl, aryl or mixed alkyl-aryl halogensilanes are used and, most preferably, dimethyldichlorosilane is used. It is also possible to use the corresponding esters of the silanes. The latter do not produce optimally stable products but have the advantage that hydrogen halide is not split off when they are used thereby eliminating the necessity for deacidification.
The organophilic or hydrophobic fillers produced by the aforesaid processes find many uses, for instance, as free-flowing agents in powder systems, as fillers in special coating compounds, e.g., paint primers, and as fillers for plastics and elastomers such as natural and synthetic rubber.
However, fillers for use in silicone rubber have to meet additional requirements such as freedom from halogens and possession of a greater thickening effect than the above-mentioned hydrophobic products. Fillers for use in silicone rubber are, therefore, preferably formed by treating the oxides with organosiloxanes. To this end, a number of processes have become known for "coating" natural and synthetic fillers such as, for instance, silicic acid or materials containing the same. In these processes, the finely divided filler is mixed with a liquid siloxane or treated in a fluidizing bed with a finely dispersed siloxane, whereby more or less strongly adherent coatings are obtained on the filler surface. In order to obtain the optimum degree of adhesiveness between the filler particles and the agent which imparts hydrophobic properties, a chemical bond between the two is necessary. The prior art processes have not been acceptable because of apparatus limitations and the time required to adequately mix the components.
A prior art process (British Pat. No. 932,753, U.S. Pat. No. 2,803,617) makes use of an acid or like material as a reaction catalyst in order to economically carry out the reaction of pyrogenically produced silicic acid with siloxanes such as D.sub.4 octamethylcyclotetrasiloxane, for example.
According to another known procedure, the reaction is carried out without pressure but the treatment of the silicic acid is effected in batches and involves extended residence times, e.g., 3-4 hours in certain stages of the process. Thus, a continuous process in an economical manner is hardly possible.
The object of the invention is to provide an arrangement which enables highly dispersed fillers to be produced in an economically and technically feasible manner, which enables fillers distinguished by their stability and optimal hydrophobic and organophilic properties to be produced and which enables fillers of particular suitability for use as additives in silicone rubbers to be produced.